Two-photon absorbing materials are useful for optical applications such as switches, laser amplifiers, and optical down-converters. Rubidium vapor, which has sharp spectral features, has been used or envisioned as a two-photon absorbing material for these applications. However, rubidium vapor may be difficult to use in practice.
Spectral properties of atoms, particularly metals, are extremely sensitive to the environment in which they are placed. It is well-known that absorbance and emission spectra of atoms are broadened and shifted by interaction with other atoms. The close interaction of two metal atoms can also lead to quenching of fluorescence properties, again demonstrating a change of spectral properties.
Small nanoparticles may be synthesized with a single lanthanide metal atom at the center, with a screening set of transition metal atoms, as depicted in FIG. 1 of Japanese Journal of Applied Physics 46, pp L83-L85 (2007), see also U.S. Pat. No. 7,695,641).
Fluorescence measurements performed on these nanoparticles show the emission spectrum of the Eu atom to be very narrow, even when the nanoparticles are incorporated into a polymer at high loading. This indicates that the Eu atoms remain isolated by the surrounding Al and O atoms.
The drawback to these nanoparticles is that the electronic isolation of the central Eu atom in high concentrations is presumed to be dependent on the coordination of the Eu (or other lanthanide) by the six oxygen atoms.